[ubuntu] USB death?

On my system I have 18 USB-3 hard disk attached. Worked fine over the past years. They are coected via USB-3.0 hubs to the USB port on the computer.

A month ago, one hard drive was not recognized. I thought on a hardware failure.
I swapped harddisks first and then 4 hard disks were not recognized anymore.
I swapped hubs, but could not figure out any useful information of that.

Above were only USB-3 ports.

Then my printer / scaer stopped to be recognized (USB-2)
Clearly that has nothing anymore with the hard disks or USB hubs to do!

I changed the motherboard.

Still the same.

It must therefore be something on the system. How can I track that down?

uname -a
Linux ronald-desktop 3.16.0-34-generic #47~14.04.1-Ubuntu SMP Fri Apr 10 17:49:16 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux